Engine systems may be configured with a boosting device, such as a turbocharger, for providing a boosted aircharge and improving peak power outputs. Therein a turbine is rotated using energy from an exhaust flow, the turbine then driving a compressor which delivers a boosted aircharge to the engine intake. To improve exhaust emissions, engine systems may also be configured with exhaust gas recirculation (EGR) systems wherein at least a portion of the exhaust gas is recirculated to the engine intake. For example, the EGR system may be a low-pressure EGR system (LP-EGR) that recirculates exhaust gas from downstream of an exhaust turbine to upstream of an intake compressor. EGR benefits include an increase in engine dilution, decrease in exhaust emissions, and improvements in fuel economy, especially at higher levels of engine boost.
Introduction of (low pressure) EGR upstream of the compressor requires the compressor inlet pressure to be reduced, so that the EGR can be pulled in from the engine exhaust system, downstream of the turbine. The low pressure at the compressor inlet generates a pressure differential across the EGR passage that enables the desired EGR flow to be drawn in. The low compressor inlet pressure may be achieved by throttling the compressor inlet with an additional throttle known as the air intake system (AIS) throttle. One example of such a system using multiple throttles is shown by Ulrey et al. in U.S. Pat. No. 8,161,746. However, the inventors herein have recognized potential issues with such an approach. As one example, the low pressure at the compressor inlet increases the potential for compressor surge. In addition, durability concerns may be raised if oil from the turbocharger shaft seal is pulled into the turbocharger. Further still, the need for an additional throttle increases component cost as well as complexity in coordinating the control of the additional throttle with the main intake throttle.
In one example, some of the above issues may be addressed by a method for an engine comprising: recirculating an amount of compressed air from downstream of a charge air cooler to a compressor inlet via a venturi, and using vacuum generated at the venturi to draw EGR into the compressor inlet. In this way, recirculation flow through a venturi can be advantageously used to generate vacuum for drawing in EGR at a pre-compressor location while also addressing surge.
For example, an engine system may be configured with a compressor recirculation passage that recirculates cooled compressed air from downstream of a charge air cooler to a compressor inlet via a continuously variable compressor recirculation valve (CRV). A venturi may be positioned in the compressor recirculation passage downstream of the CRV such that compressed air is recirculated to the compressor inlet upon flowing through the venturi, the flow generating a vacuum at the venturi. An EGR passage including a continuously variable EGR valve for recirculating exhaust residuals from the engine exhaust to the compressor inlet may be coupled to the compressor recirculation passage at the venturi. An opening of the CRV may be adjusted to provide a margin to surge. The vacuum generated by the surge mitigating compressor recirculation flow may be advantageously used to enhance the pressure differential in the EGR passage. An opening of the EGR valve may then be adjusted based on the opening of the CRV (or the venturi vacuum level) to meet the EGR flow demand. For example, as the venturi vacuum available from the compressor recirculation flow increases, a smaller EGR valve opening may be required to meet the EGR flow demand.
In this way, EGR flow can be enhanced using vacuum induced using compressor recirculation flow. By drawing in the EGR to the compressor inlet using assistance from venturi vacuum, the need for pre-compressor throttling, including the need for a dedicated throttle is reduced. By enabling EGR to be drawn in without reducing the pressure at the compressor inlet, a margin to surge is also improved. By using compressor recirculation flow to assist in drawing in EGR, surge control and EGR control are concurrently achieved. Overall EGR benefits can be provided over a larger engine operating window while boosted engine performance is also improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.